The present invention is directed generally to the detection of an output termination of a input measurement device and more particularly to an output termination auto detect circuit for an input measurement device, such as current measurement probe, coupled to the input termination of a host measurement instrument.
Oscilloscopes are measurement instruments that provide a visual representation of a measured signal in a time versus amplitude display on a display device, such as a cathode ray tube or liquid crystal matrix display. The input channels of the oscilloscope have input termination resistances which are generally 1 megohm or 50 ohms depending on bandwidth performance. Measurement probes, such as voltage and current probes, are coupled to the input channels of the oscilloscope and have output terminations that are compatible with the input terminations of the input channels. As the bandwidth of oscilloscopes have increased, it became necessary to provide both types of input terminations for the input channels of oscilloscopes. The input termination of the input channel is selectable by the user for the particular measurement probe being used. This can result in terminating the output termination of a measurement probe in the wrong input termination of the input channel resulting in inaccurate signal measurements.
U.S. Pat. No. 5,493,211, titled “Current Probe”, describes a current probe measurement system having an A6302 Current Probe, an AM503 Current Probe Amplifier and a TM500 Power Supply that are manufactured and sold by Tektronix, Inc, the assignee of the current invention. The current probe includes a ring-shaped core of magnetic material defining an aperture through which a current carrying conductor is inserted. A multi-turn winding is wrapped around one leg of the core. A thin-film semiconductor Hall Effect device is disposed in the core, perpendicular to the lines of flux in the core. A bias source provides power to the Hall Effect device with the output of the device coupled to the input of an amplifier. The output of the Hall Effect amplifier is coupled to one side of the multi turn winding with the other side of the winding coupled across a load resistor to an input of a scaling output amplifier. The output of the amplifier is coupled via a coxial cable to the input of an oscilloscope.
The current carrying conductor and the winding around the core act as a transformer for inducing an AC current into the winding of the current probe representative of the AC current in the conductor. The Hall Effect device generates an output in response to the magnetic flux in the core representative of the DC or low frequency current in the conductor. The Hall Effect amplifier generates a current that is opposite to the direction of the magnetic field created by the current in the conductor. The current output coupled across the load resistor is representative of the current in the conductor.
FIG. 1 illustrates the scaling amplifier 110 of the AM503 Current Probe Amplifier 112 operating as an output termination auto detect circuit 114. The scaling amplifier 110 is coupled to the input of the oscilloscope 116 input via a coaxial cable 118. The output termination ROUTPUT TERMINATION 120 of the scaling amplifer 110 is the input termination of the oscilloscope. The input to the scaling amplifier 110 is coupled to receive and VOFFSET ADJUST signal for estimating the output termination of the scaing amplifier. The scaling amplifier 110 has a transfer function, gain (Av) between VOFFSET ADJUST and VSOURCE, and an output impedance, ZOUT, 122. An analog-to-digital converter (ADC) 124 is coupled to the output of the scaling amplifier for converting the output of the amplifier to digital values representative of the voltage at the output. Applying a known input in the form of a VOFFSET ADJUST and knowing the gain (Av), the impedance, ZOUT, and the output voltage of the scaling amplifier 110, the value of output termination ROUTPUT TERMINATION can be calculated using the below equation.ROUTPUT TERMINATION=(VOUTPUT*ZOUT/(Av*VOFFSET ADJUST−VOUTPUT)  (1)
The above described output termination auto detect circuit provides a rough estimate of the output termination of the current probe amplifier system. It can distinguish between a 1 Megohm termination and a shorted termination. However, it cannot accurately determine if the current probe amplifier system is coupled to a 50 ohm termination. This is because the transfer function of the scaling amplifier can vary from device to device, which adds uncertainty to the calculation of ROUTPUT TERMINATION. For example, it is known that the above output termination auto detect circuit will provide a false 50 ohm termination indication when a 50 ohm termination is coupled to the input of a 50 ohm terminated input of an oscilloscope.
What is needed is an output termination detection circuit that more accurately detects the output termination of the measurement probe (i.e the input termination of the host measurement instrument). The output termination detection circuit provides an indication when the output termination of the measurement probe is improperly terminated.